Cements, mortars and concretes are widely used throughout the industrial world. Among the more important applications of these materials are those found in the construction field for the building of highways, pavements, aircraft runways, parking lots, floors, foundations, bridges, houses, industrial building, silos, docks and many other structures. The characteristics of the end products vary considerably according to the specific application involved, and many mixes have been formulated to produce the desired effects. An acceleration of the normal rate of strength development at an early age often is sought as a major consideration in an important segment of the general field. The benefits from this property are diversified, such as earlier removal of forms, composition for low ambient temperature, and earlier placement in service of a structure or repair of a structure.
Labor and material cost increases often make repair rather than placement of deteriorating concrete surfaces increasingly attractive. For example, street and highway maintenance is a national disaster in the USA today. According to Federal estimates, fully one-third of the nation's bridges are in a deteriorated condition and a multitude of pot holes need repair in our highways. Part of the problem results from greater demands now imposed on traffic lanes than were designed into these structures when they were built only a few years ago. Rapid repair is especially necessary in high traffic areas of pavements and bridges, where high premium is placed on minimum lane downtime for repair, low cost, least hazard to motorists and repair workers, and the smallest damage to various passing vehicles.
Many patch materials developing early high strengths have been suggested including the following groups: basically portland cements of different types, other chemical-setting cements, hydrated calcium sulfate (gypsum), thermoplastics, thermosetting materials, bituminous materials, sulfur, and composites, with or without various additives. Each has advantages and disadvantages.
Type III portland cement has been quite extensively used for its early high strength characteristics, but the setting time of this cement is slower than often is desired and there is high shrinkage, especially if the water content is not kept low. Also used are high-alumina cements, of the non-calcium aluminate type, but they too have excessive shrinkage, and permit water and deicer penetration, which causes deterioration as a result of corrosive action, or cracking because of ice expansion during the setting stage, which sometimes causes disintegration of the repaired structure.
Polyester resins usually are hardened with very small quantities of catalysts, which often is difficult to control, and some of the epoxy resins bond poorly with damp surfaces. Some of the magnesia-phosphate compositions, sulfur, and solvent asphalt mixtures emit disagreeable vapors which usually are toxic or inflammable.
Many materials and methods have been proposed to control the setting rates of portland cements, including both retarders to extend pot life during emplacement, and accelerators to develop early high strength. Some are added alone, while others are combined with other admixtures.
Included in the list of chemicals that accelerate the hardening of mixtures of portland cement and water are some of the soluble chlorides, hydroxides, carbonates, sulfates and various organic compounds such as triethanolamine.
Calcium chloride probably is the most commonly used accelerator for portland cements in spite of its tendency to increase the corrosion rate of any steel reinforcement which is used, especially in prestressed concrete, because of possible stress corrosion of the prestressed steel. Shrinkage or expansion often are encountered with these mixtures as a result of the alkali aggregate reaction, and these factors often are difficult to control.
In a few publications, sodium silicate or "water glass" has been recommended as an accelerator for the setting of Portland Cement in cases where a "false set" is desired in special cases such as the quick patching of a hole in a cement tank. The "false set" is considered to be an undesirable property in cement accelerators, used in a conventional manner, and is employed only as an expedient in case of an emergency. Because of this bad reputation, there appears to be no reputable study reported concerning the conditions that are necessary in conventional applications for the effective use of silicates in controlled early high strength mortars and concrete.